1. Field of the Invention
The invention is concerned primarily with an improvement in a drive transmission of the gear type which couples an engine to a driven load at any selected one of a plurality of drive ratios. Most particularly, the invention is concerned with such a drive transmission wherein both the input clutch and the output clutch of said drive transmission are fluid operated by an automatic operating control means. Specifically, the invention provides a trigger mechanism which operates an overspeed inhibitor coupled to the control means and the output clutch for overriding the control means by changing the pressure of fluid applied to the output clutch when the input shaft rotates above a preselected rate. Further, the invention is concerned with the particular trigger mechanism utilized and with the use of said particular trigger mechanism in a drive transmission to serve as the actuator in such an overspeed inhibitor and in other situations as well.
2. Prior Art
Multi-speed drive transmissions are well known to the prior art. More particularly drive transmissions for powered vehicles and the like which have change speed gears which may be shifted to provide a selected one of a plurality of drive ratios and have clutches at both the input and output end thereof whereby the gears may be decoupled from both the driving engine and the driven shaft and may then be braked to a stop to facilitate shifting are known. In such apparatus brakes are generally provided at both the input and output ends of the gearing section and a fluid control system, generally a fast acting pneumatic control system is provided which responds to movement of the operators shift lever by automatically operating the clutches and brakes in a programmed sequence which assures smooth shifting and avoids damage. Thus it is known to provide such a drive transmission wherein on movement of the operator shift lever to a selected setting the control system simultaneously disengages both clutches and applies both brakes. The control system then energizes fluid operated means for making the appropriate gear interconnections and then sequentially releases the input brake and the output brake. The input clutch is engaged and finally the output clutch is engaged to re-establish a driving connection between the engine and the driven elements. The delay between release of the input and output brakes assures that abutting teeth in the gearing section engage easily at relatively low speeds and under only light loading. Such systems are described in great detail in each of U.S. Pat. Nos. 3,508,450; 3,667,538; and 3,667,309 which are assigned to the assignee of the present application. The description in said three United States Patents is hereby incorporated herein by reference thereto.
A problem can arise in drive transmissions for coupling an engine to a driven load such as for example those transmissions described in detail in the above referenced patents due to erroneous movement of the shift lever to signal a transmission ratio change which would seriously overspeed the transmission and the engine. Similarly, engine and transmission overspeeding can be caused by a malfunction of the control apparatus for the transmission. For example, the basic concept of the transmissions of the above discussed patents relies on shifting from one speed ratio to another with all internal rotation stopped. Because of this, transmission and engine overspeeding can be caused by improper ratio selection. That is, with all internal rotation stopped the transmission can be shifted into any drive ratio be it proper or improper. Then, when the transmission engages as by brake release and clutch engagement the transmission and engine can suddenly be accelerated into an overspeed condition. This is clearly a serious problem which can deleteriously affect both the transmission and the engine. The present invention is concerned with use of a centrifugal trigger device which causes the drive connection to be slipped until the speed of the input shaft of the transmission reduces to an acceptable level and then re-engages the connection. The connection which is slipped and then re-engaged is the output clutch at the rear of the transmission since this serves to slip the drive connection to both the transmission and the engine.